This invention relates to digital cellular telephone systems and, more particularly, to base stations using directional antenna arrays.
One cellular communication system, known as time-division multiple access (TDMA), divides the radio spectrum into plural channels. Each channel is sub-divided into time slots that can be assigned to a different subscriber. Mobile phones operating under the United States"" D-AMPS standard, which is also known as IS54 or IS136, utilize a syncword to assist in demodulation and decoding of a transmitted signal to a mobile station. The syncword consists of a pattern of known symbols.
The syncword is transmitted at the beginning of each time slot that has been allocated for transmission from the base station to the mobile station. Systems that utilize the D-AMPS standard always transmit at a constant power level in all time slots on the same carrier frequency. A mobile station can decode the allocated slot using both the known symbols at the beginning of its allocated slot and the beginning of a subsequent slot, known as a postamble. As a result, demodulation and decoding may take place in forward or backward order. This may be done using the algorithms described in Dent et al., U.S. Pat. No. 5,335,250 or U.S. application Ser. No. 08/218,236, the specifications of which are incorporated by reference herein.
The use of a postamble syncword can thus be used to improve performance. Its use is possible because the D-AMPS downlink is TDM, rather than TDMA. TDM is a continuous transmission having phase, bit-timing and power level continuity across slot boundaries. Thus, all signals allocated to different slots at the same carrier frequency are transmitted by the base station at equal power. The power level must be the greatest of the three base station transmit power levels that the three mobiles"" occupying the three slots per carrier require. Schaeffer, U.S. Pat. No. 4,866,710 discloses a system that concentrates calls within each frequency to reduce the number of frequencies carrying calls. Alternatively, Dent U.S. Pat. No. 5,579,306 discloses grouping calls onto the same carrier that have similar downlink power requirements and then minimizing the carrier power levels to the greatest of the power levels needed in any slot. Thus, the prior art describes various examples of attempting to maintain syncword transmissions coherent with the preceding slot to allow its use by receivers as a postamble.
Cellular systems are continually designed to obtain capacity improvement. One possible method relies on reduction of antenna beamwidth which effectively increases the number of azimuthal sectors available to construct a frequency re-use plan. The site re-use pattern can advantageously be smaller. For example, instead of a seven site by three sector re-use plan, a three-site by twelve sector re-use plan might be used. This brings one-third of the total frequency channels into use within a site instead of only one-seventh. This provides a capacity gain of 7/3.
Increasing antenna beam directivity can result in more sectors than carrier frequencies. Then it is no longer possible to give each sector a unique frequency. To avoid the use of the same frequency at the same time in different sectors of the same site, each frequency may be divided into time slots to create a large number of unique time slot-frequency pairs that can be divided among the sectors. The construction of time and frequency re-use plans is described in Dent U.S. Pat. Nos. 5,539,730; 5,566,168, 5,619,503 and 5,594,941, and Honda et al. U.S. Pat. No. 5,555,271, the specifications of which are hereby incorporated by reference herein.
Charas et al. U.S. Pat. No. 5,548,813, the specification of which is incorporated by reference herein, describes how different time slots in a TDMA system are transmitted using different groups of elements of a phased array to achieve different effective isotopically radiated power levels for different time slots.
The above described prior art transmits different time slots in different directions or at different power levels. This violates the TDM signal continuity across slots that current D-AMPS mobile stations assume. Mazur et al. U.S. patent application Ser. No. 08/887,726 discloses the use of multiple antenna arrays for preserving the signal continuity across time-slots.
Adaptive channel assignment (ACA) is also used to obtain capacity improvements. ACA is an automatic way of achieving re-use partitioning. Channels may be re-used on a tighter grid when they are transmitted at less than maximum power to mobiles nearer the center of cells and not at a cell edge. A 1.7:1 gain in capacity over fixed re-use plans is evidenced in simulations. However, this gain is conditional upon the use of dynamic power control. If power levels are not adapted dynamically for each call, then the gain through using ACA drops to about 1.3:1. Again, dynamically varying the power level between slots may also violate the TDM signal continuity that current D-AMPS mobile stations assume.
Thus, there is a need for a method for maintaining signal continuity across slots when slots are transmitted in different directions and at different power levels.
In accordance with the invention there is provided a method and system that uses overlapping slot transmissions using phased arrays.
Broadly, there is disclosed herein a time-division multiple access (TDMA)base station for preserving across-slot signal continuity of signals transmitted in different directions on a given frequency. The base station includes a multi-directional antenna for radiating signals. A processor is operatively coupled to the antenna for generating a first data signal for a first time slot of a TDMA frame including a first pre-determined symbol pattern and a second pre-determined symbol pattern, for generating a second data signal for a second time slot of the TDMA frame including the second pre-determined symbol pattern, and for communicating the first data signal and the second data signal to the antenna such that the first data signal is radiated in a first direction and the second data signal is radiated in a second direction. The first data signal is modulated at a first phase for transmission in the first time slot at a first power level in the first direction on the given frequency. The second data signal is modulated at a second phase for transmission in the second time slot at a second power level in the second direction on the given frequency.
It is a feature of the invention that the first power level is substantially equal to the second power level.
It is another feature of the invention that the first phase is at a 90xc2x0 phase difference with respect to the second phase.
It is a further feature of the invention to provide a modulator for modulating the first data signal and the second data signal on the given frequency.
It is still another feature of the invention that the processor performs the modulating of the first data signal and the second data signal on the given frequency.
It is still a further feature of the invention that the power level of the first data signal is ramped down from the first power level to zero after the second pre-determined symbol pattern is transmitted in the first direction, and the power level of the second data signal is ramped up from zero to the second power level before transmission of the second pre-determined symbol pattern in the second direction.
It is still an additional feature of the invention that the first direction and the second direction are two of four pre-determined directions.
It is disclosed in accordance with another aspect of the invention a TDMA cellular base station for transmitting signals in different time slots of a TDMA frame in different directions while preserving across-slot signal continuity as seen by intended receivers. The base station includes an antenna for forming a plurality of directional beams. A first signal generator modulates a first data signal including a first pre-determined symbol pattern, first data symbols and a second pre-determined symbol on a given radio frequency channel to be radiated during a first time slot at a first power level in a first direction using the antenna. The first signal generator smoothly ramps a first data signal level down from the first power level to zero after transmission of the second pre-determined symbol pattern. A second signal generator modulates a second data signal including at least the second pre-determined symbol pattern and second data symbols on the given radio frequency channel to be radiated during a second time slot at a second power level in a second direction using the antenna. The first and second time slots overlap during transmission of the second known symbol pattern and the second symbol generator ramps up from zero to the second power level prior to transmitting the second pre-determined symbol pattern.
It is a feature of the invention that the first signal generator modulates the same data symbols as the second signal generator while performing the down-ramping.
It is another feature of the invention that the second signal generator modulates the same data symbols as the first signal generator while performing the up-ramping.
It is an additional feature of the invention that the first and second signal generators are adapted to cause the first signal to be at a 90xc2x0 phase difference with respect to the second data signal when the first and second data signals are radiated from the antenna in the first and second direction during the up-ramping, second pre-determined pattern transmission and down-ramping.
There is disclosed in accordance with still another aspect of the invention a method of preserving across-slot signal continuity of signals transmitted in different directions on a given frequency from a TDMA base station, comprising the steps of generating a first data signal for a first time slot of a TDMA frame including a first pre-determined symbol pattern and a second pre-determined symbol pattern; generating a second data signal for a second time slot including the second pre-determined symbol pattern; modulating the first data signal on the given frequency at a first phase; modulating the second data signal on the given frequency at a second phase; transmitting the first data signal from the base station in the first time slot at a first power level in a first direction, on the given frequency at the first phase; and transmitting the second data signal from the base station in the second time slot at a second power level in a second direction, on the given frequency at the second phase.
There is disclosed in accordance with yet another aspect of the invention a method of transmitting signals in different time slots of a TDMA frame in different directions in a TDMA cellular base station using a multi-beam directive antenna array while maintaining across-slot signal continuity as seen by intended receivers of the time slot, including the steps of transmitting a first signal in a first direction at a first power level during a first time slot, including transmitting a postamble comprised of pre-determined symbols at an end of the first time slot, and transmitting a second signal in a second direction at a second power level during a second time slot, including transmitting a preamble at the beginning of the second time slot comprised of the pre-determined symbols, and overlapping the transmission of the postamble.
It is a feature of the invention to include driving elements of the antenna such that the first signal and the postamble are received at the first power level by an intended receiver lying in the first direction, and the preamble and the second signal are received at the second power level by a different intended receiver lying in the second direction.
It is another feature of the invention to include selecting the first and second power levels such that each of the intended receivers receives at least a minimum acceptable signal level while minimizing a total power radiated by the antenna array.
It is another feature of the invention to include selecting the antenna element drive signals to cause a directional transmission to smoothly change from the first direction to the second direction during the overlapping transmission of the preamble and postamble.
It is a further feature of the invention to continue transmission of the first signal in the first direction for a period after completing transmission of the postamble while the power level of transmission of the first signal is smoothly ramped-down to zero.
It is yet another feature of the invention to include commencing transmission of the second signal in the second direction for a second period before starting transmission of the preamble while the power level of transmission of the second signal is smoothly ramped-up from zero.